import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class code4 {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head ListNode类 the head node
     * @return ListNode类
     */
    public ListNode sortInList (ListNode head) {
        // 归并排序

        //特殊情况
        if(head == null || head.next == null) return head;

        ListNode left = head;
        ListNode mid = head.next;
        ListNode right = head.next.next;

        //找到mid和right
        while(right!=null && right.next!=null) {
            left = left.next;
            mid = mid.next;
            right = right.next.next;
        }
        left.next = null;
        return merge(sortInList(head),sortInList(mid));
    }

    //合并两段链表
    ListNode merge(ListNode head1,ListNode head2) {
        if(head1 == null) return head2;
        if(head2 == null) return head1;

        if(head1.val<=head2.val) {
            head1.next = merge(head1.next,head2);
            return head1;
        } else {
            head2.next = merge(head1,head2.next);
            return head2;
        }

    }
}